1. Field of the Invention
The present invention relates to a filter catalyst for purifying exhaust gases containing particulates, such as those emitted from diesel engines, and a manufacturing method thereof.
2. Description of the Related Art
Regarding gasoline engines, harmful components in the exhaust gases have been reduced securely by the strict regulations and the technology developments capable of coping with them. However, regarding diesel engines, the regulations and the technology developments have been advanced less compared to those of gasoline engines because of the unique circumstances that the harmful components are emitted as particulates (i.e., particulate matters, such as carbonaceous fine particles, sulfuric fine particles like sulfates, and high-molecular weight hydrocarbon fine particles, hereinafter collectively referred to as “PMs”).
As exhaust-gas purifying apparatuses having been developed so far for diesel engines, the followings have been known. For example, the exhaust-gas purifying apparatuses can be roughly divided into trapping (or wall-flow) exhaust-gas purifying apparatuses and open (or straight flow) exhaust-gas purifying apparatuses. Among these, clogged honeycomb structures made from ceramic (i.e. diesel PMs filters, hereinafter referred to as “DPFs”) have been known as one of the trapping exhaust-gas purifying apparatuses. In the DPFs, the honeycomb structures are clogged at the opposite openings of cells in a checkered manner alternately, for instance. The DPFs comprise inlet cells clogged on the downstream side of the exhaust gases, outlet cells clogged neighboring the inlet cells and clogged on the upstream side of the exhaust gases, and filter cellular walls demarcating the inlet cells and the outlet cells. In the DPFs, the exhaust gases are filtered by the pores of the filter cellular walls to collect PMs.
In the DPFs, however, the pressure loss increases as PMs deposit thereon. Accordingly, it is needed to remove deposited PMs regularly to recover the DPFs, somehow. Hence, when the pressure loss increases, deposited PMs have been burned with burners or electric heaters conventionally to recover the PMs. However, the more the deposition mass of PMs is, the higher the temperature rises in burning deposited PMs. Consequently, there might arise cases that the DPFs are damaged by thermal stress caused by the temperature rise.
Hence, continuously regenerative DPFs have been developed recently. In the continuously regenerative DPFs, a coating layer comprising alumina is formed on the surface of the filter cellulr walls of the DPFs, and a catalytic ingredient such as platinum (Pt) is loaded on the coating layer. In accordance with the continuously regenerative DPFs, since the collected PMs are oxidized and burned by the catalytic reaction of the catalytic ingredient, it is possible to regenerate the DPFs by burning PMs simultaneously with or successively after collecting PMs. Moreover, since the catalytic reaction occurs at low temperature, and since it is possible to burn PMs when they are collected less, the continuously regenerative DPFs have an advantage that the thermal stress onto the DPFs is so less that the DPFs are inhibited from being damaged.
For example, Japanese Unexamined Patent Publication (KOKAI) No. 9-173,866 discloses such a continuously regenerative DPFs with a filter cellular wall on which a porous coating layer comprising activated alumina with a particle diameter larger than the average pore diameter of the filter cellular wall is formed, and the inside of the pore is coated with activated alumina with a particle diameter less than the average pore diameter of the filter cellular wall and further, catalytic ingredients are loaded thereon. In accordance with the continuously regenerative DPFs, it is possible to make the pressure loss relative small to the increase of specific surface area of the porous coating layer.
Moreover Japanese Unexamined Patent Publication (KOKAI) No. 9-220,423 discloses such a continuously regenerative DPFs whose filter cellular wall exhibits a porosity of from 40 to 65% and an average pore diameter of from 5 to 35 μm, and whose coating layer is formed of a porous oxide. In the porous oxide, particles with a diameter less than the average pore diameter of the filter cellular wall occupy 90% by weight or more. When such a porous oxide with a large specific surface area is coated on DPFs, it is possible to form the coating layer not only on the surface of the filter cellular walls but also on the inner surface of the pores. Moreover, on the condition that the coating layer is coated in a fixed amount, it is possible to make the thickness of the coating layer thinner. Accordingly, it is possible to inhibit the pressure loss from enlarging.
Moreover, Japanese Unexamined Patent Publication (KOKAI) No.6-159,037 discloses a continuously regenerative DPF whose coating layer is further loaded with an NOx sorbent. With the arrangement, NOx can be sorbed in the NOx sorbent. Consequently, when a reducing agent such as diesel oil is sprayed, it is possible to reduce the sorbed NOx to purify.
However, continuously regenerative DPFs have a problem with the limited activity. Specifically, it is impossible to increase the loading amount of catalytic ingredient because the coating amount of the coating layer is limited in view of the pressure loss. On the other hand, when a large amount of catalytic ingredient is loaded on a thin loading layer, the loading density of catalytic ingredient is enlarged so that the granular growth of catalytic ingredient occurs at high temperatures. As a result, continuously regeneration DPFs are deteriorated in terms of the durability.
For example, in the technique mentioned in Japanese Unexamined Patent Publication (KOKAI) No.9-173,866, the coating layer is formed by wash-coating the slurry prepared from a blended powder of alumina with a large particle diameter and a small diameter on the DPFs. In this manner, however, there are some large particles entering into the pore, and therefore, it has a possibility that these particles, together with small particles, may clog the pores, which leads the increase of the pressure loss. When the amount of coating is reduced in order to inhibit the increase of the pressure loss, the loading density of catalytic ingredient is enlarged so that the granular growth of catalytic ingredient occurs. As a result, the durability of the continuously regenerative DPFs deteriorates.
Moreover, in the continuously regenerative DPFs whose coating layer contains an NOx sorbent, when the amount of coating is insufficient, the reaction such as solid solution of the NOx sorbent to a filter substrate material leads the problem that the ability of the NOx purification is deteriorated. As the amount of coating is increased, the reaction of the NOx sorbent to a filter substrate material can be suppressed, but, because of the choke of pores the probability of catalytic reaction of the NOx sorbent to exhaust gases is decreased, and the ability of NOx purification is also deteriorated. Moreover, the pressure loss rises and the PM collecting rate decreases.
By the way, in the circumstances that the PMs are collected in the pores of the filter cellular walls, the oxidation reaction of PMs proceeds smoothly because the probability of catalytic reaction of catalytic ingredients to PMs is high and the heat retaining property is well. Moreover, it is possible to estimate the deposition amount of PMs by detecting the pressure loss because the pressure loss increases sensitively according as the PMs are collected. Therefore, by executing the regeneration process such as streaming high temperature exhaust gases when the pressure loss exceeds a reference value, it is possible to burn PMs in the deposition amount within a reference amount, and thus to prevent the temperature of the continuously regenerative DPFs from rising high at the time of burning.
However, under a low temperature or the condition that a large amount of PMs are exhausted continuously, since the speed of deposition for PMs becomes larger than that of oxidation reaction, PMs are deposited and form a layer along the filter cellular wall. Since a deposition layer formed on the surface of the filter cellular wall by PMs is oxidized only on the boundary between the filter cellular wall and the deposition layer in ordinary conditions, a clearance is formed between the deposition layer and the filter cellular wall, and the rate of the pressure loss becomes relatively small with respect to the increase of the deposition amount, thereby lowering the sensitivity for the detection of the pressure loss. Therefore, a method for estimating the amount of PMs deposition by detecting the pressure loss has the error that the estimated PMs deposition amount differs much from a real deposition amount.
Once a clearance is formed between the deposition layer and the filter cellular wall, the surface of deposition layer is not brought into contact with the catalytic ingredient. Therefore, the rate of oxidation for PMs lowers, and the deposition is promoted on the surface, and the amount of deposition becomes extremely large. As a result, when the temperature of exhaust gas rises, the deposited PMs are burned all at once, and the temperature of the continuously regenerative DPFs become high, so that the problems that the catalytic ingredient is deteriorated by the granular growth of catalytic ingredient or the damages of filter substrate by dissolution occurs.